This article is from the Aerobics Fitness FAQ, by firstname.lastname@example.org (Robles) with numerous contributions by others.
Actually, you're *almost* always burning fat at one rate
or another, but you burn most when your body is in its aerobic
range. A good rule of thumb is that after 20 minutes in your
aerobic zone, you will be burning more fat than carbos. Covert
Bailey, in "Smart Exercise", states that you will be burning
fat after only twelve minutes of aerobic exercise. If you
can increase your aerobic activity to 30 minutes or longer,
you will be burning a larger percentage of calories from fat.
There is still some disagreement as to which is better - longer
duration at lower intensity, or shorter duration at higher
intensity. If you are limited in time, then the higher intensity
will maximize your aerobic benefits in a shorter amount of time.
If you can work for a longer duration at a lower intensity, you
will decrease your chance of injury. If you are interested
in decreasing the amount of fat on your body, the idea is to
use more calories than you take in. Your muscles will continue
to burn fat after both aerobic and anaerobic (muscle training)
(from Michael G. Kurilla [email@example.com. virginia.edu])
This is perhaps the most common question raised by individuals
exercising for the purpose of either weight loss or simply
weight control. This stems from the recognition that aerobic
exercise is a significant adjunct to any weight loss program,
that is diet plus aerobic exercise produces more weight loss
than diet alone. In addition, the weight lost with exercise
tends to be a higher percentage of fat.
Exercise can be grouped into three broad levels of intensity,
mild, moderate, and high. Mild intensity is a comfortable walking
pace and can be sustained almost indefinitely, moderate intensity
is equal to an average cardiovascular conditioning workout (able
to talk, but not sing) and can be sustained (in a trained individual)
for upwards of 3 - 4 hours, and high intensity is not
able to talk and can only be sustained for 30 - 45 minutes.
Based on recent and very detailed research studies, in terms of
absolute fat burning, a moderate intensity workout burns the most
fat. At a heart rate equal to about 75% of max, fat burning will
approach 0.5 grams - 1.0 grams of fat per minute. There is a weight
dependence with the lower end referring to a 100 pound individual
and the upper end to a 200 pound person. As the duration continues
(greater than 1 hour), fat burning can increase slightly (another
At a mild intensity, the majority of calories expended (85 - 90%)
are fat calories, but the absolute level is only about 60% of the
moderate intensity. At high intensity levels, fat burning declines
to a level of about 65% of the moderate pace, as sugar burning
supplies the rest. The high rate of sugar burning exhausts the
limited sugar supply in muscles and causes muscular failure.
The only caveats for the above burn rates are that these numbers
are derived from individuals who were already aerobically trained
and were conducted in the AM before breakfast. Less fit individuals
are known to burn less fat and more sugar (part of aerobic
conditioning is greater reliance on fat burning for energy).
Exercising after a meal will tend to promote more sugar burning.
Consumption of sugar during an exercise session will also tend to
retard fat burning in favor of the sugar. These numbers were derived
from cycling and so the absolute numbers can be increased if
exercises that involve more muscle groups are utilized (running,
rowing, etc.). From peak energy production rates for various
exercises, rowers might reach about 40% higher.